Synchronization in a utility power grid is a critical issue for the purpose of control and operation when distributed power generators are connected to the grid. The synchronization includes determining a phase angle of 3-phase voltage signals in the grid. Usually, the grid voltage signal deviates from the ideal condition and is distorted due to additive noise, frequency variation, voltage unbalance, and harmonic components. As a result, it is desired to provide synchronization that minimizes such distortions. Similarly, fast detection of the frequency and the phase angle of the voltage signal are critical for reconnecting, or disconnecting distributed generators to manage power demand.
The unbalanced 3-phase voltage signal includes a positive, a negative and a zero phase sequences. Usually, the synchronization requires detecting the phase angle of the positive sequence of the voltage signal. A number of conventional methods for detecting the phase angle were developed.
A zero-crossing method is a simplest technique. However, that method is sensitive to the disturbances of the voltage signal. Also, the phase angle is determined only at zero-crossing points, and, hence, that method has low dynamic performance.
A phase locked loop (PLL) method can be used with a synchronous rotating reference frame. However, performance of the PLL method degrades in the presence of voltage unbalance because a double frequency component is introduced due to the existence of the negative phase sequence.
Several methods have been developed to handle the unbalance based on the separation of the positive and negative sequences through the application of symmetrical component transformation. Another method uses a fixed weight vector to estimate the frequency and phase angle of the voltage signal. However, that method is not adaptive. The weights are selected heuristically, and therefore, in some situations the method performs inaccurately.